1. Field of the Invention
The present invention relates to a thin-film semiconductor device that is formed on a substrate with an insulating surface such as a glass substrate. In particular, the invention relates to a thin-film transistor with a high breakdown voltage, which is formed using a crystallized thin-film semiconductor formed on the substrate.
2. Description of the Related Art
In order to drive an active-matrix flat-panel display that is employed as a display for image information or text information, attention has been paid to the use of thin-film transistors (TFTs) that are formed on an insulating substrate. In addition, the thin-film transistors have been put to practical use as devices that constitute logic circuits, etc. in various types of electronic equipments.
FIG. 1A and FIG. 1B show an example of the thin-film transistor that has widely been put to practical use. FIG. 1A is a plan view, and FIG. 1B is a cross-sectional view taken along line I-I in FIG. 1A. As is shown in FIG. 1A and FIG. 1B, a thin-film transistor 10 is configured such that a semiconductor island 20 that is formed of, e.g. an amorphous silicon thin film is provided on an insulating substrate 11. A channel region 12 and source/drain regions 13, 14 are formed in the semiconductor island 20. A gate insulation film 15 and a gate electrode 16 are formed on the channel region 12. The source/drain regions 13, 14 are provided with source/drain electrodes 17, 18, and the source/drain regions 13, 14 are connected to other electronic devices by metal wires 19.
In the thin-film transistor 10 with the above-described structure, an electric field concentrates at an edge portion 23 between an upper surface 21 and a side surface 22 of the channel region 12, leading to a decrease in breakdown voltage of the thin-film transistor. Besides, with development in fine patterning technology, the effect of the side surface 22 of the channel region 12 has become unignorable. That is, there is a problem that substantially different field-effect transistors are formed in parallel at the upper surface 21 and side surface 22.
There is known a constitution wherein a ring-shaped gate structure is formed on a semiconductor single-crystal substrate. Jpn. Pat. Appln. KOKAI Publication No. 10-223898 (Reference Document 1), for instance, discloses the ring-shaped gate structure. With this structure, a device with a large gate width can be formed with a small occupation area. The increase in gate width can reduce the drain current density. Making use of this structure, an operation with a large current can be secured when a high voltage is applied, and breakdown voltage characteristics can be enhanced. Jpn. Pat. Appln. KOKAI Publication No. 11-97697 (Reference Document 2), for instance, discloses a ring-shaped gate structure relating to a thin-film transistor that is formed on an insulating substrate.
The ring-shaped gate MOSFET disclosed in Reference Document 1 is formed on the semiconductor single-crystal substrate. Thus, the channel region is formed on one single crystal without fail, and no crystal grain boundary is, in principle, present in the channel region. Therefore, the device characteristics are very stable, and non-uniformity in characteristics is considered to be small. However, at present, no method is known to form a single-crystal thin film over the entire surface of an inexpensive insulating substrate of, e.g. glass. In this situation, when a thin-film transistor is to be formed on an insulating substrate, an amorphous semiconductor thin film or a polycrystalline semiconductor thin film, which is formed on an insulating substrate, is used in usual cases.
Amorphous semiconductor thin films have widely been used since they can be formed at low temperatures and are suited to mass production. However, the mobility of carriers in the amorphous semiconductor thin film is considerably lower than that in the polycrystalline semiconductor thin film. It is thus difficult to achieve high-speed operations with the amorphous semiconductor thin film. For semiconductor devices that require high-speed operations, a polycrystalline semiconductor thin film, which is formed using a method such as a laser crystallization method, is used at present.
The polycrystalline semiconductor thin film that is formed by the laser crystallization method, however, has a very small crystal grain size, compared to the size of an operation region of the semiconductor device. Even if the transistor disclosed in Reference Document 2 is formed using the polycrystalline semiconductor thin film, a great number of crystal grain boundaries occur at random in the channel region of the transistor. The crystal grain boundaries function to prevent the flow of carriers. Consequently, the mobility of carriers in the channel region decreases, and a considerable non-uniformity occurs in electrical characteristics among devices.
The present invention aims at solving the above-described problems. Specifically, an object of the invention is to provide a thin-film transistor that is formed on an inexpensive insulating substrate such as a glass substrate, and is capable of high-speed operations and has small non-uniformity in characteristics among devices. Another object of the invention is to provide a thin-film transistor that is hardly susceptible to device destruction even if a high voltage is applied, and can have a sufficiently high breakdown voltage. Still another object of the invention is to provide a thin-film transistor that is free from the effect of a parasitic transistor that forms on a side wall of a Si (silicon) island.